U.S. Pat. No. 4,454,064 discloses a method for preparing pentaerythritol phosphate, which comprises reacting pentaerythritol with POCl3 in dioxane solvent at 75-125xc2x0 C. The pentaerythritol phosphate thus prepared can be used as an intermediate for synthesizing a polyurethane flame retardant and a plasticizer. This preparation method will also generate HCl gas and a residual solution of excessive POCl3.
U.S. Pat. No. 4,478,998 discloses a synthesis of an amino-s-triazine salt of a phosphoric acid having the following formula: 
when X and Xxe2x80x2 in the formula are amino, said salt is melamine salt of bis-(pentaerythritol phosphate) phosphoric acid. The synthesized amino salt can be used as a flame retardant additive for certain polymer compositions. In Example 1 of said patent, melamine reacts with acid chloride of bis-(pentaerythritol phosphate) phosphoric acid in water to obtain said melamine salt. Said acid chloride of bis-(pentaerythritol phosphate) phosphoric acid is synthesized through the following reaction formula: 
HCl gas and a residual solution of excessive POCl3 will also be generated in the abovementioned reaction.
The present Invention provides a method for preparing a melamine salt of bis-(pentaerythritol phosphate) phosphoric acid, which comprises preparing bis-(pentaerythritol phosphate) phosphoric acid, and reacting melamine with the obtained bis-(pentaerythritol phosphate) phosphoric acid to form a melamine salt thereof. The preparation of bis-(pentaerythritol phosphate) phosphoric acid according to the present invention has the following characteristics:
1. The method uses P2O5 as a reactant.
2. The method uses a mechanochemical synthesis and uses a ball mill as a reactor.
3. The method uses an alkyl benzene, such as toluene or xylene, as a solvent. The alkyl benzene may have one or two identical or different alkyls having 1 to 5 carbons.
4. The solvent used in the method is at room temperature or pre-heated to 50-150xc2x0 C.
5. The method uses metal halide MX2, such as MgCl2 etc., as a catalyst, wherein M=Mg, Zn, or Al; and X=Cl, or Br. The weight ratio of the catalyst to pentaerythritol is 1:99 to 5:95.
Compared to the conventional process, the present invention has the following three advantages: (a) no generation of waste gas of HCl; (b) free of a waste aqueous solution generated from neutralization of HCl waste gas; and (c) avoiding handling of a residue solution containing unreacted POCl3.
The reaction between melamine and bis-(pentaerythritol phosphate) phosphoric acid according to the present invention can be carried out in a ball mill or a conventional mechanical agitation reactor, which comprises ball-milling or mechanically mixing a mixture of melamine, bis-(pentaerythritol phosphate) phosphoric acid, and a solvent at room temperature to 100xc2x0 C. Suitable solvents include (but not limited to) water, acetonitrile (CH3CN), and a mixture of the solvent used in the preparation of bis-(pentaerythritol phosphate) phosphoric acid with acetonitrile. Preferably, said solvent is pre-heated to 50-100xc2x0 C. and thus there is no heating while ball milling said mixture. More preferably, said solvent is a mixture of toluene with acenonitrile preheated to 50-80xc2x0 C., acetonitrile that has been pre-heated to 80xc2x0 C. or boiling water, most preferably is boiling water.
The present invention discloses a method for preparing melamine salt of bis-(pentaerythritol phosphate) phosphoric acid, which comprises preparing bis-(pentaerythritol phosphate) phosphoric acid, and reacting melamine with the obtained bis-(pentaerythritol phosphate) phosphoric acid to form a melamine salt thereof. Several factors affecting the synthesis of said bis-(pentaerythritol phosphate) phosphoric acid are discussed in the following:
The molecular structure unit of phosphorus pentoxide is P4O10, usually represented by P2O5. Phosphorus pentoxide is a very strong dehydration agent, and is liable to react with water to form phosphoric acid.
P2O5+3H2Oxe2x86x922H3PO4
Furthermore, it can grab H2O from reactant molecules to form metaphosphoric acid and related inorganic or organic material, e.g.
P2O5+H2SO4xe2x86x92SO3+4HPO3P4O10+H2Nxe2x80x94C(O)xe2x80x94C(O)xe2x80x94NH2xe2x86x92NCxe2x80x94CN+4HPO3P2O5+CH2xe2x95x90C(CH3)xe2x80x94C(O)xe2x80x94OH+CH3OHxe2x86x92CH2xe2x95x90C(CH3)xe2x80x94C(O)OCH3+2HPO3
P2O5 reacts with ethyl ether to form triethyl phosphate:
P2O5+3(C2H5)2Oxe2x86x922(C2H5O)3Pxe2x95x90O
The strong reactivity of phosphorus pentoxide makes the selection of the reaction solvent greatly restricted. The inventors of the present invention have tried using a solvent such as n-hexane, diethyl phosphate, and toluene, etc. for performing the reaction, in which the reaction of using n-hexane as a solvent is not ideal and has an extremely low yield. When diethyl phosphate is used as a solvent, the reaction has a very good yield. However, since the properties of diethyl phosphate are too close to the properties of the product bis-(pentaerythritol phosphate) phosphoric acid, they are difficult to be separated. Therefore, toluene is selected as a reaction solvent.
Pentaerythritol can be dissolved in diethyl phosphate solvent, therefore, after addition of P2O5, the reaction can take place at a reaction temperature of 90xc2x0 C. While not dissolving in toluene, pentaerythritol will turn into a molten state when the temperature rises to 90xc2x0 C. and can react with P2O5. Since pentaerythritol in its molten state has an extremely high viscosity and is difficult to agitate, the reaction yield is not high. Therefore, how to achieve an ideal agitation for pentaerythritol to have a sufficient contact with P2O5 is a key factor in increasing the yield. Pentaerythritol does not dissolve at the boiling point of n-hexane at 68xc2x0 C. and has no change in state at this temperature, this could be a reason why it does not react with P2O5 in n-hexane.
Pentaerythritol does not react with P2O5 in toluene solvent at a temperature lower than 70xc2x0 C. under traditional stirring process. When the temperature increases to 90xc2x0 C., pentaerythritol can undergo a phosphate esterification reaction. The reaction time is about 10 hours. When the temperature rises to 105xc2x0 C., the reaction time can be reduced to 6 hours.
The reaction yield and purity can be increased when, prior to the reaction, pentaerythritol is ground in toluene in a ball mill.
Based on the above studies 1xcx9c3, the key factors affecting the method of the present invention include: heating of the solvent, material with a fine particle size, and complete mixing in the reaction. Therefore, a ball mill is considered as a reactor to achieve a complete grinding, mixing and thermal insulation in the reaction. A suitable reaction time is 4-20 hours.
When 2 moles of pentaerythritol reacts with 1.5 mole of P2O5, 1 mole of bis-(pentaerythritol phosphate) phosphoric acid and 3.5 moles of water will be generated. Since 3 moles of water will react with 1 mole of P2O5 to from 2 moles of phosphoric acid, and a consumption of P2O5 occurs. Therefore, an excessive amount of P2O5 should be used for all the pentaerythritol phosphate alcohol to be converted to bis-(pentaerythritol phosphate) phosphoric acid. A suitable mole ratio of pentaerythritol to P2O5 is 4:3 to 1:2.
The reaction between melamine and bis-(pentaerythritol phosphate) phosphoric acid according to the present invention can be carried out in said ball mill or a conventional mechanical agitation reactor, preferably in a ball mill. The reaction can be carried out using acetonitrile or water as the solvent. According to the present invention, when the conventional mechanical mixing is used and acetonitrile is used as a solvent, the reaction can be completed in 6 hours by reflux. In the ball mill process where acetonitrile which is pre-heated to 80xc2x0 C. is used, the reaction time is 7.5 hours. In the ball mill process, if toluene is still used as a solvent in the reaction for forming melamine salt after the preparation of bis-(pentaerythritol phosphate) phosphoric acid, the result is poor. After the completion of the preparation of bis-(pentaerythritol phosphate) phosphoric acid in the ball mill process, if the original solvent (400 ml) is retained and another 100 ml of acetonitrile, which is pre-heated to 80xc2x0 C., is added as a solvent in the reaction for forming melamine salt, the reaction can be carried out smoothly and the reaction time is also 7.5 hours. Furthermore, if the remaining solvent is poured out and boiling water is added as a solvent in the reaction for forming melamine salt, the reaction time can be greatly reduced to 15-60 minutes. The obtained melamine salt of bis-(pentaerythritol phosphate) phosphoric acid has a good heat resistance, only about 10xcx9c20% of which will be decomposed at 200xcx9c300xc2x0 C., and about 40% of which will remain at a temperature exceeding 600xc2x0 C.
The present invention uses P2O5 to replace POCl3, and a mechanochemical synthesis process to overcome lack of a suitable solvent. Thus, the present invention has the following advantages: no formation of hydrochloric acid, no waste solution, environmentally friendly, without the need of heating in the reaction, high yield, Increased purity, and capability of recycling solvent.